feat: add article search index endpoint and category export.zip

- Add GET /api/articleIndex returning the article search index (listSearchIndex)
- Add GET /api/category/export.zip returning a kategorien.zip with a
  Windows-1252 CSV (depth-first, Ebene N columns) plus one AVIF per category image
  (content-addressed, deduplicated) using jszip

docs/api/routes/articleIndex.md

@@ -0,0 +1,33 @@
+# `GET /articleIndex`
+
+Protected. Requires a valid session cookie (`credentials: 'include'`).
+
+List article search index
+
+Returns every article reachable via the company's category tree.
+Designed for client-side search / autocomplete preload.
+Auth via company access.
+
+
+## Response `200`
+
+Search index
+
+```json
+{
+  "articles": []
+}
+```
+
+## Errors
+
+| Status | Description |
+|--------|-------------|
+| 400 | No company selected — `{"error":"…"}` |
+| 401 | Unauthorized — `{"error":"…"}` |
+| 403 | Not authorized — `{"error":"…"}` |
+| 404 | Company not found — `{"error":"…"}` |
+
+---
+
+_Generated from `@openapi` JSDoc. Do not edit by hand — run `npm run docs:api`._